Thermionic valve amplifier



Nov. 12, 1940. -E WH T 2,221,122 THERMIONIC VALVE AMPLIFIER Filed Oct. 21, 1938 FR EaumcY INVENTORS E. L. C, WHITE G. HARKER ATTORNEY Patented Nov. 12, 1940 UNlT-ED STATES PATENT OFFICE THERMIONIC VALVE AMPLIFIER Great Britain" Application October 21, 1938, Serial No. 236,146

In Great Britain October 22, 1937 V The present invention relates to thermionic valve amplifiers of the type inwhich negative feedback is employed to reduce amplitude dis- I tortion and to render the amplification less dependent upon changes in polarising Voltages and valve characteristics. I

When amplifiers of this type are employed for the amplification of signals comprising a wide band of frequencies, such as television signals, it is found that stray capacity associated with the output circuit reduces the amplification at the higher frequencies. It has been proposed in the specification of British Patent No. 465,118 to 1 shunt the cathode resistance employed for feed back purposes with a condenser, the time constant of the resistance and condenser beingso chosen that it is approximately equal to the time constant of the anode load resistance and the stray capacity; This has the effect of maintaining the amplification constant to a higher frequency than it would be if no condenser were used, but for very high frequenciesthe impedance of the condenser falls to such a low value that it is no longer large compared with the inverse of the mutual conductance of the valve which condition is necessary if the amplification is to be maintained substantially constant for the very high frequencies encountered-in television signals.

, It is the object of the present invention to overcome this difficulty so that the amplificationmay be maintained substantially constant .up-to a higher frequency than would otherwise be pos- 35 sible.

' According to the invention a thermionic valve amplifier is provided of the kind comprising a thermionic valve having an output impedance which is substantially equivalent to a resistance shunted by a capacity and a feedback impedance including a capacity in shunt and so arranged as to afford negative feedback so as to increase the frequency range over which substantially uniform amplification occurs and wherein there 45 is provided in series with the capacity included in said feedback impedance an inductance the value of which is so chosen as to further increase the frequency range over which substantially uniform amplification occurs. 1

50 More specifically, the invention comprises a 2 Claims, ,(01. 129-471) over which substantially uniform amplification occurs and wherein there is provided in series with the capacity shunting said feedback resistance an inductance the Value of which is so chosen as to further increase the frequency range over which substantially uniform amplification occurs.

In order that theinvention may be clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawing in which:

Figure 1 illustrates in conventional form an amplifier constructed in accordance with the preferred embodiment of the invention with the feedback impedancein the cathode circuit of the valve, the sources of the anode current, gridvoltage, etc., being 0 l tted for simplicity, and

Figure 2 is an expanatory figure.

The amplifier as shown in Figure l comprises a thermionic valve I having four electrodes, namely, the cathode 2, control grid 3, the screen grid 4 and the anode :5. The cathode is connected to earth via resistance Re shunted by inductance Lo and condenser Cc inv series, which constitute the negative feedback impedance Zc- The control grid 3 is connected to earth via the source of input voltage V1 which is to be amplified. The screen grid 4is connected to a source of constant voltage, not shown, whilst the anode is connected to the output resistance Ra, which is returned to earth; The condenser Ca, in parallel with resistance Ra represents stray capacity, which may bedue, for example, to further apparatus receiving the amplified output voltage V2 developed across resistance Ra. With a sinusoidal input voltage V1, let the alternating T s I 2c 1+yzo =yV1 We also have VzilchZu (2) where Za is the impedance between the anode 5 and earth and k is a constant representing the proportion of the current leaving the cathode and reaching the anode through the screen.

From Equations 1 and 2 we have 1 HFQR) Therefore we have l 1 +jwcRc) Now V2 can be made substantially constant over a range of frequencies in known manner by making CcRc='C'aRa, but at high frequencies since but this condition is clearly not satisfied when the impedances of Co falls: to equality with and for frequencies greater than where Vzwill therefore fall below its constant value for lower frequencies.

By inserting an inductance in series with the condenser Co the denominator in the above expression for V2 can be caused to vary substantially inversely with frequency up to frequencies higher than and it can be shown that particularly good'results are obtained when" the inductance has a value inserted in the cathode circuit, while curve (b) is that characteristic obtained when an inductance of magnitude 1 1 9 2%?) is connected in series with the condenser Cc.

Although the invention has been described with reference to screen grid valves, it will be obvious that pentode valves or any other high impedance valves having screen grids may equally well be used.

We claim:

1. A'thermionic valve amplifier of the kind comprising a thermionic valve having an output impedance which is substantially equivalent to a resistance shunted by a capacity and a feedback impedance including a capacity in shunt and so arranged as to afford negative feedback so as to increase the frequency range over which substantially uniform amplification occurs, the limiting frequency of said range being 010/21, and wherein there is provided in series with the capacity included in said feedback impedance an inductance thevalue of which is so chosen as to further increase the frequency range over which substantially uniform amplification occurs, the limiting frequency of said latter range being approximately Zola/211', the value of said inductance being equal to (g) Where Cc is the value of the condenser in shunt to the feed-back impedance and g is the mutual conductance of the valve.

2. A thermionic valve amplifier of the kind comprising a thermionic valve having an output impedance in its anode circuit which is substantially equivalent to a resistance shunted by a capacity and a feedback resistance in the cathode circuit of said valve so connected that the voltage set up across said resistance is effectively applied in antiphase to the control electrode of said valve, said feedback resistance being shunted by a capacity so as to increase the frequency range over which substantially uniform amplification occurs and wherein there is provided in series with the capacity shunting said feedback resistance an inductance the value of which is so chosen as to further increase the frequency range over which substantially uniform amplification occurs, the value of said inductance being equal to ERIC LAWRENCE CASLING WHITE. MAURICE GEOFFREY HARKER. 

